Just when Southern Californians have begun to pat themselves on the backs for having the
world's stiffest earthquake building codes, there now comes news of "supershears." These are not competitors to Supercuts, but a newly discussed type of earthquake known for the incredibly high velocities by which it travels -- and compacts the earth.
New Scientist writers Richard Fisher describes "one that slipped at such blistering speeds that the rip in the Earth overtook its own seismic waves. This created the earthquake equivalent of a sonic boom, capable of striking anything in its path like a hammer blow."
According to Fisher's article, which appeared online Wednesday, evidence
shows that this little-understood phenomenon occurs more often than
previously suspected and that a "superhighway" of supershear faults
grids the planet where 60 million people live. Although a brief moment
during a 1979 Imperial Valley earthquake seemed to fit the description
of a supershear, such quakes had long been relegated to the realm of
theory -- until Turkey's 1999 Izmit quake. That 7.6 earthquake was
measured as spreading five kilometers per second. (Five kilometers is about 3.1 miles. The speed of sound
is 343 meters per second.)
Fisher says that
some geologists now believe the 1906 San Francisco earthquake may have been
a supershear. More ominously, he notes that not only is California's
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San Andreas Fault part of the supershear superhighway, but says that even the state's well-fortified buildings may be no match
for the "mach fronts" generated by supershears, and quotes a Caltech
scientist as noting that less-rigorously built structures five
kilometers outside of L.A. would be especially at risk.